The other performances were sound (Samuel L Jackson's hair included) with Jamie Bell particularly outstanding as the nervy Griffin. His performance adds to the frantic energy of the film and every scene without him is the poorer for it.
While the pacing and energy of the film keeps you glued to your seat, it is only on reflection that I realised how unsatisfying the story ultimately was - leaving me with an "is that all?" kind of feeling. The answer, of course, will probably be 'No' as this film seems to have been made with the idea of sequels firmly in sight.
All in all - a tightly made film, with the scenes inside the Colosseum worth the price of admission alone - if only they'd cast someone else in the lead role!